1. Field of the Invention
This invention relates to a light beam scanning system, and more particularly to an improvement in a light beam scanning system for recording information employing a rotating mirror as a beam deflector.
2. Description of the Prior Art
In light beam scanning systems employing a laser beam, various beam deflectors are used for making rasters, such as light beam deflectors utilizing an electro-optical effect or acoustooptical effect, vibrating mirrors of galvanometer type and rotating polygonal mirrors. The rotating mirrors are advantageous in that a large angle of deflection can be obtained, a high resolution of image can be effected and it does not have a spectroscopic effect. However, rotating mirrors have a defect in that the start positions of the rasters are liable to fluctuate due to the errors in manufacturing of the rotating mirrors and/or the fluctuation in the speed of rotation of the motor used for driving the rotating mirrors.
In order to manufacture rotating mirrors having highly accurately cut facets, considerably high technique of manufacture is required. Further, such rotating mirrors become extraordinarily expensive. In order to prevent the fluctuation in the speed of rotation of a motor, a high performance expensive motor must be used together with a complicated feed-back control.
It has been known in the art to eliminate the jitter in the direction of rasters in light beam scanning systems without increasing the accuracy in the manufacturing of the rotating mirrors or eliminating the fluctuation in the speed of rotation of a motor. In Laser Focus issued Jan. 1, 1966 p.9-12, a laser television camera system developed by Parkin Elmer is disclosed. In this system, an information read-out device employs a rotating mirror and a photodetector located in the vicinity of the starting point of rasters formed by the rotating mirror. The output signal of the photodetector is used as a synchronizing signal to operate a display device such as a cathode ray tube. This system disclosed in Laser Focus has a defect in that the quality of image is not sufficient when the rotation of the rotating mirror is not synchronized with the video clock signal.